Westwood T1800 Ride on Mower 2008 stalls when lifting the clutch/brake ?

My Westwood T1800 Mower starts and runs just fine but as soon as you lift the brake/clutch pedal under the left foot the engine dies but does catch again if the clutch/brake is depressed before the engine fully stops. The mower has a hydrostatic drive which does occasionally move the mower in reverse and forward drive but reluctantly as the engine dies. I have checked the fuses and the airfilter etc as per the manual and they are all ok Can anyone please help. Thanks